6th 6 Months Flashcards
One advantage of DC motors over AC motors is that a DC motor
offers better speed control
In DC motors, eddy currents can cause
overheated brushes
Which of the following cells has the shortest service life
Lead-acid
In the electroplating process, the object to be plated is made the
cathode
In arc welding, a “reversed polarity” exists when the electrode is _____ and the work is _____.
positive;negative
If you double the distance between two magnetic poles, the magnetic force is
reduced to 25%
A solenoid used for closing a valve or releasing a brake is usually made with a _____-shaped magnet.
C-T
Mounting a solenoid from the end
improves its performance
When a relay operates, its NC contacts
open and remain open
The movable magnetic arm of a relay is called the
armature
Which part of a DC generator makes electrical contact between the brushes and the coils of wire in the armature
Commutator
In DC generators, the number of pulses during each rotation of the armature _____ with an increase in the number of ______.
increases;field poles
Commutation in a DC generator keeps the current to the output terminals
in the same direction at all times
A DC generator that supplies a nearly constant potential difference between terminals is called a ____ generator.
compound
Eddy currents in a DC genrator cause ____ losses.
magnetic
Commutation is the
reversal of current in a motor’s armature
How much work do you accomplish if you carry a 50 lb object 10 ft horizontally?
0 ft-lb
The field windings in a DC compound motor are connected so that _____ with the armature
one is in parallel and one is in series
The armature current and the _____ determine the torque in a DC motor.
field strength
The only kind of DC motor to run faster under a high load than under a low load is a ____ motor.
differential compound
Which part of DC armature is made of copper bars that are insulated from each other?
Commutator
A winding that doubles back on itself in a drum armature is called a ____ winding.
lap
If the full-load current in a DC armature is 100A and the resistance between the brushes is 0.05 ohms the copper loss is
500W
Which type of power loss varies with the speed of a DC armature and the volume of iron in the core?
Hysteresis loss
A bright spark produced while a DC machine is running indicates an _____ in the armature.
open circuit
A time-delay feature can be added to a magnetic overload relay in the form of an
oil dashpot
In a shunt relay, the size of the air gap between the coil and the armature determines the
voltage needed to operate the contactor
The current at which a series lockout relay will close can be changed by adjusting the
size of the air gap
The device that protects a relay by extinguishing the arc formed when contacts open is a
magnetic blowout coil
A disc brake is designed to be mounted on the _____ of a motor.
end bell
An adjustable-voltage motor controller varies the potential difference applied to the motor’s
armature
Controllers are rated by duty as ___ or ____.
continuous;intermittent
Which overload relay is used only in AC circuits?
magnetic
Changes in ambient temperature will change the tripping characteristics of a thermal overload relay unless the relay is
temperature compensated
A DC motor that requires frequent starting, stopping, reversing, and changes speed usualy has a
drum-type controller
A rectifier changes
AC to DC
A pi filter network consists of ___ and ____.
two capacitors;one inductor
One advantage that a mercury-vapor diode has over a vacuum-tube diode is that the mercury-vapor diode
produces a higher power output
Most mercury-vapor rectifiers have a time-delay relay to
protect the diode
Semiconductor failures caused by high temperatures usually occur
in a fraction of a second
Another name for a silicon controlled rectifier is a
thyristor
The part of an SCR that either allows or prevents the flow of electricity is called the
gate
In an AC circuit, an SCR conducts during _____ of the cycle
either half but not both halves
An SCR in an AC circuit can be turned on earlier in the half-cycle in order to _____ the ____.
increase;power to the load
Increasing the effective current and power level from an SCR controller to a DC motor requires
narrower intervals between pulses
A sudden drop in the potential difference between terminals usually indicates an _____ circuit in the field winding of a DC _____.
open;generator
Before installing a new field coil in a machine you should
determine the polarity of the new coil
If silver relay contacts become slightly roughened or pitted, you should
do nothing
To ensure proper communication in a reversible motor, you should set the brushes _____ degrees from electrical neutral.
0
A blackened, scratched, or dirty commutator should be cleaned with a
a high-quality cleaning stone
In Boolean algebra, all logic functions can be expressed by equations based on three operations:
and, or, not
For an AND gate, the output is TRUE if input A is ___ and input B is ____.
TRUE;TRUE
A solid-state logic switch must be able to
indicate which state is at its input
The AND function in positive logic is equivalent to the ___ function in negative logic.
OR
If CMOS logic is used in a circuit with TTL logic, the input is usually
5V
An application requires an R-S flip-flop that will trigger when S goes HIGH and reset when R goes HIGH. The required flip-flop can be made up of _____ gates.
two NOR
The advantage of using an R-S flip-flop with a mechanical switch is that the flip-flop ______ the output when the switch changes
makes changes in
The output of a clocked flip-flop will change only when the clock pulse is
present
If you need to use a sine wave to control the frequency of a logic clock, you can use a _______ to produce sharp clock pulses.
Schmitt trigger
What is the hexadecimal equivalent of the binary numeral 11100?
1C
The problem with ripple counters is that
propagation delay is cumulative
A device commonly used for converting parallel data to serial data is the
multiplexer
A half-adder can be made of an ____ gate and an ____ gate.
AND;Exclusive OR
Large memories are typically accessed by
matrix addressing
Unlike a RAM, a ROM
stores data that do not change
An output interface contains a _____ that produces a ____ signal in response to the output of the logic system.
transistor;high voltage
